1. Field of the Invention
The present invention relates to a gas sensor and a method for controlling the same for measuring oxides such as NO, NO2, SO2, CO2, and H2O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles, and inflammable gases such as CO and CnHm.
2. Description of the Related Art
Recently, an oxygen sensor is widely known, for measuring a specified gas component, for example, oxygen, in which the voltage or the current is controlled to apply it to an oxygen pump based on the use of an oxygen ion-conductive member composed of a solid electrolyte of ZrO2 so that oxygen is pumped in or pumped out under a predetermined diffusion resistance to measure a limiting current obtained during this process (see, for example, Japanese Laid-Open Patent Publication No. 8-271476).
Another sensor is also known, in which a proton pump is constructed by using an oxygen-proton ion-conductive member so that the limiting current is measured on the basis of the same principle as that used in the oxygen sensor to measure H2 and H2O.
A NOx sensor 200 as shown in FIG. 15 is also known, which is used to measure, for example, NOx as a specified gas component.
The NOx sensor 200 is operated as follows. That is, a measurement gas is introduced into a first hollow space 204 via a first diffusion rate-determining section 202. A first oxygen-pumping means 212, which is constructed by an inner pumping electrode 206, an oxygen ion-conductive member 210, and an outer pumping electrode 208, is used to pump out or pump in oxygen contained in the measurement gas in such a degree that the measurement gas is not decomposed. Subsequently, the measurement gas is introduced into a second hollow space 216 via a second diffusion rate-determining section 214. A second oxygen-pumping means 226, which is constructed by a measurement gas-decomposing electrode 218 arranged in the second hollow space 216, an oxygen ion-conductive member 220, and a reference electrode 224 arranged in a reference air section 222, is used to pump out oxygen which is produced by decomposition effected by the catalytic action of the measurement gas-decomposing electrode 218. The sensor measures the value of current which is required to pump out the oxygen.
In other words, the foregoing gas sensors are operated such that the specified gas component is detected by using the ionic current, and the concentration of the predetermined gas is ensured in the internal space of the sensor by controlling the ionic current value.
However, the gas sensors as described above are disadvantageous as follows. That is, when the concentration of the measurement gas is low, the pumping current is decreased. As a result, it is difficult to perform the detection in some cases, and the accuracy is greatly deteriorated by the external electric noise in other cases.
For example, in the case of the NOx sensor 200 shown in FIG. 15, when the NOx concentration in the measurement gas is 10 ppm, the signal level is in a degree of about 0.05 xcexcA. As a result, it is difficult to perform the detection. Further, it is feared that the measurement accuracy is greatly deteriorated due to the external electric noise.
In order to accurately control the oxygen concentration in the second hollow space 216, the present applicant has suggested a NOx sensor 250 as shown in FIG. 16. The NOx sensor 250 comprises an auxiliary pump 252 which is provided for the second hollow space 216. The controlled oxygen concentration in the first hollow space 204 is corrected so that the current, which flows through the auxiliary pump 252, is constant (see, for example, Japanese Laid-Open Patent Publication No. 9-113484 and European Patent Publication No. 0 807 818 A2).
In the case of the NOx sensor 250, the auxiliary pumping current is not more than several xcexcA which is small. Therefore, it has been revealed that the controlled oxygen concentration in the second hollow space 216 cannot be corrected at the desire of a user in some cases.
On the other hand, in the case of the sensors as described above, the limiting current is utilized to control the concentration of the gas component and measure the concentration thereof. Therefore, if the limiting current value is changed, the output is changed. In this context, for example, the limiting current value involves dispersion among individual sensors. At present, in order to correct the dispersion among individual sensors, a shunt resistor is provided, or a voltage divider resistor is provided.
FIG. 17 shows an arrangement of such a countermeasure. When the current, which flows to an oxygen pump 260, is detected by using a current-detecting resistor Ra, the current supply from a variable power source 262 to the oxygen pump 260 is shunted by the aid of an adjusting resistor Rb (shunt resistor).
For example, when the gas sensor has a large limiting current, the shunt resistor Rb is decreased so that the amount of shunt is increased. Thus, the amount of current, which is detected by the current-detecting resistor Ra, is decreased to be a predetermined value. On the contrary, when the gas sensor has a small limiting current, the amount of shunt is decreased so that the current, which is detected by the current-detecting resistor Ra, is adjusted to be the predetermined value.
Another method is also available such that the voltage, which is generated between the both terminals of the current-detecting resistor Ra, is subjected to voltage division by using a voltage divider circuit to obtain a predetermined output voltage.
However, when the foregoing methods (the shunt resistor system and the voltage divider resistor system) are adopted, one extra lead wire is required, in accordance with which it is necessary to use a multiple terminal connector system for connecting the control circuit and the sensor, resulting in a problem concerning the cost.
The present invention has been made considering the problems as described above, an object of which is to provide a gas sensor and a method for controlling the gas sensor which make it possible to highly accurately measure a predetermined gas component while scarcely being affected by the electric noise or the like.
Another object of the present invention is to provide a gas sensor and a method for controlling the gas sensor which are advantageous in view of the production cost and which make it possible to compensate the dispersion among individual sensors without increasing the number of terminals, in addition to the requirement described above.
A gas sensor according to the present invention comprising:
a main pumping means for pumping-processing oxygen contained in a measurement gas introduced from external space, comprising solid electrolyte contacting with said external space, and an inner pumping electrode and an outer pumping electrode formed on inner and outer surfaces of said solid electrolyte; and
a measuring pumping means for decomposing a predetermined gas component contained in said measurement gas after being pumping-processed by said main pumping means by the aid of a catalytic action and/or electrolysis, and pumping-processing oxygen produced by said decomposition via said outer pumping electrode of said main pumping means, wherein:
a concentration of oxygen is controlled and/or the predetermined gas component is measured by allowing a pulse-shaped current to flow through said measuring pumping means;
the gas sensor further comprising:
a electromotive force-measuring circuit for constantly measuring the electromotive force corresponding to a difference between an amount of oxygen produced by said decomposition of said predetermined gas component and an amount of oxygen contained in a reference gas;
a frequency control means for controlling a frequency of said pulse-shaped current corresponding to a difference between an the electromotive force measured by said electromotive force-measuring circuit and a comparing voltage; and
a measuring circuit for at least converting the frequency of the pulse-shaped current into a concentration of said predetermined gas component.
Accordingly, the concentration of oxygen is controlled and/or the predetermined gas component is measured by supplying the current from the current supply means to the measuring pumping means.
Usually, the following method is adopted in relation to the measurement of the predetermined gas component. That is, a constant voltage is applied to the measuring pumping means to measure the predetermined gas component by detecting the value of current flowing through the measuring pumping means depending on the amount of oxygen during this process. In such an ordinary method, the detected current value is extremely small. Therefore, a problem arises in that the measurement tends to be affected by the external electric noise.
On the contrary, according to the present invention, the current, which is supplied to the measuring pumping means, has the pulse waveform having the constant crest value. Further, the frequency of the pulse waveform is controlled. In such a procedure, the use of the pulse waveform makes it possible to obtain the crest value which is higher than those obtained when the current is supplied in the direct current form. Therefore, it is possible to allow the system to be scarcely affected by the electric noise or the like. The use of the frequency as the measured value makes it possible to increase the output dynamic range (frequency region) with respect to the inputted electromotive force. Thus, it is also possible to improve the measurement sensitivity.
It is preferable for the gas sensor constructed as described above that a power source for the current supply means is a constant voltage power source, and a resistor is connected in series to a current supply line to the measuring pumping means. In this arrangement, the voltage from the constant voltage power source is allowed to have a pulse-shaped voltage waveform by the aid of the current supply means. The current, which is supplied to the measuring pumping means, is a pulse-shaped current which has a crest value obtained by dividing the crest value of the voltage by the resistance value of the series resistor. In other words, the crest value of the pulse-shaped current supplied to the measuring pumping means can be adjusted by changing the resistance value of the series resistor. In this arrangement, it is preferable that the series resistor is selected or adjusted depending on performance of a sensor element.
Accordingly, it is possible to compensate the dispersion (the dispersion concerning the crest value and the output) among the individual sensors without increasing the number of terminals, which is advantageous in view of the production cost.
The gas sensor according to the present invention is preferably used for a NOx sensor for measuring NOx in a measurement gas.
In another aspect, a gas sensor according to the present invention comprising:
a electromotive force-measuring circuit for constantly measuring the electromotive force corresponding to a difference between an amount of oxygen produced by said decomposition of said predetermined gas component and an amount of oxygen contained in a reference gas;
a duty ratio control means for controlling a duty ratio of said pulse-shaped current corresponding to a difference between an the electromotive force measured by said electromotive force-measuring circuit and a comparing voltage; and
a measuring circuit for at least converting the duty ratio of the pulse-shaped current into a concentration of said predetermined gas component.
According to the present invention, the current, which is supplied to the measuring pumping means, has the pulse waveform having the constant crest value. Further, the duty ratio of the pulse waveform is controlled. Also in this aspect, the use of the pulse waveform makes it possible to obtain the crest value which is higher than those obtained when the current is supplied in the direct current form. Therefore, it is possible to allow the system to be scarcely affected by the electric noise or the like. The use of the pulse width of each waveform as the measured value makes it possible to increase the output dynamic range with respect to the inputted electromotive force. Thus, it is also possible to improve the measurement sensitivity.
It is preferable for the gas sensor constructed as described above that a power source for the current supply means is a constant voltage power source, and a resistor is connected in series to a current supply line to the measuring pumping means. In this arrangement, it is preferable that the series resistor is selected or adjusted depending on performance of a sensor element. Accordingly, it is possible to compensate the dispersion among the individual sensors without increasing the number of terminals, which is advantageous in view of the production cost.
The gas sensor according to the present invention is also preferably used for a NOx sensor for measuring NOx in a measurement gas.
In still another aspect, a gas sensor according to the present invention comprising:
a electromotive force-measuring circuit for constantly measuring the electromotive force corresponding to a difference between an amount of oxygen produced by said decomposition of said predetermined gas component and an amount of oxygen contained in a reference gas;
a crest value control means for controlling a crest value of said pulse-shaped current corresponding to a difference between an the electromotive force measured by said electromotive force-measuring circuit and a comparing voltage; and
a measuring circuit for at least converting the crest value of the pulse-shaped current into a concentration of said predetermined gas component.
According to the present invention, the current, which is supplied to the measuring pumping means, has the pulse waveform. Further, the crest value of the pulse waveform is controlled. Also in this aspect, it is possible to obtain the crest value which is higher than those obtained when the current is supplied in the direct current form. Therefore, it is possible to allow the system to be scarcely affected by the electric noise or the like. As a result, it is possible to increase the output dynamic range with respect to the inputted electromotive force. Thus, it is also possible to improve the measurement sensitivity. When the crest value is detected, it is also preferable that the current having the pulse waveform is converted into a voltage to perform the detection.
It is preferable for the gas sensor constructed as described above that a resistor is connected in series to a current supply line to the measuring pumping means. In this embodiment, it is preferable that the series resistor is selected or adjusted depending on performance of a sensor element. Accordingly, it is possible to compensate the dispersion among the individual sensors without increasing the number of terminals, which is advantageous in view of the production cost. The gas sensor according to the present invention is also preferably used for a NOx sensor for measuring NOx in a measurement gas.
In still another aspect, A method for controlling a gas sensor according to the present invention, the gas sensor comprising:
a main pumping means for pumping-processing oxygen contained in a measurement gas introduced from external space, comprising solid electrolyte contacting with said external space, and an inner pumping electrode and an outer pumping electrode formed on inner and outer surfaces of said solid electrolyte; and
a measuring pumping means for decomposing a predetermined gas component contained in said measurement gas after being pumping-processed by said main pumping means by the aid of a catalytic action and/or electrolysis, and pumping-processing oxygen produced by said decomposition via said outer pumping electrode of said main pumping means;
wherein a concentration of oxygen is controlled and/or the predetermined gas component is measured by allowing a pulse-shaped current to flow through said measuring pumping means;
wherein the method for controlling the gas sensor comprises the steps of:
measuring constantly the electromotive force corresponding to a difference between an amount of oxygen produced by said decomposition of said predetermined gas component and an amount of oxygen contained in a reference gas;
controlling a frequency of said pulse-shaped current corresponding to a difference between an the electromotive force measured by said electromotive force-measuring circuit and a comparing voltage; and
converting at least the frequency of the pulse-shaped current into a concentration of said predetermined gas component.
In still another aspect, the present invention lies in a method for controlling a gas sensor as described above, comprises the steps of:
measuring constantly the electromotive force corresponding to a difference between an amount of oxygen produced by said decomposition of said predetermined gas component and an amount of oxygen contained in a reference gas;
controlling a duty ratio of said pulse-shaped current corresponding to a difference between an the electromotive force measured by said electromotive force-measuring circuit and a comparing voltage; and
converting at least the duty ratio of the pulse-shaped current into a concentration of said predetermined gas component.
In still another aspect, the present invention lies in a method for controlling a gas sensor as described above, comprises the steps of:
measuring constantly the electromotive force corresponding to a difference between an amount of oxygen produced by said decomposition of said predetermined gas component and an amount of oxygen contained in a reference gas;
controlling a crest value of said pulse-shaped current corresponding to a difference between an the electromotive force measured by said electromotive force-measuring circuit and a comparing voltage; and
converting at least the crest value of the pulse-shaped current into a concentration of said predetermined gas component.
According to the methods for controlling the gas sensors concerning the inventions described above, it is possible to allow the system to be scarcely affected by the electric noise or the like. Thus, it is possible to measure the predetermined gas component highly accurately. Further, it is possible to compensate the dispersion among the individual sensors without increasing the number of terminals, which is advantageous in view of the production cost.
The methods for controlling the gas sensors described above are preferably applicable to a NOx sensor for measuring NOx in a measurement gas.